2007 Grand Marquis Strengthen rear suspension

HI all, I have a 2007 Grand Marquis and find with a trailer it rides low at the back. The springs seem ok. Does anyone know if there is stronger springs or if the spacers will work. Other ideas would be great as well. It is not a former service vehicle. I live in Canada.

Thanks in advance.

You could fit air bags on your GM. Many were equipped that way.

Thanks for reply. I had a Grand Marquis and a Crown Vic that both had air suspension and it failed on both of them. would prefer a stiffer spring or spacers.